Known rapid filters employ in general as a filtering screen, defined by either a lattice (woven metal cloth), a perforated metal sheet, or grids comprised of various profiled wires, or rigid piles of washers, plates or discs, without any special hydraulic profile.
The disadvantages of such construction are as follows:
With the lattice construction employing the perforated metal sheet (or perforated cylinder) and the grid, when cleaning takes place by reversing the current or back-wash cleaning, which represents the system most widely used; the fibers which may be in the liquid, slip around the passages, during cleaning a fiber will not pass the same hole or slot as the one through which it has already passed during the filtration, with the result that "felting" of the filter members takes place, necessitating them to be changed.
With all the direct acting filter elements, that is to say without any sand-bed or without a pre-layer, the passages are located at the surface and the impurities of various sizes and shapes clog those passages by collecting next to one another.
The hydraulic profiles of the filtering bodies are not designed to give a minimum loss of load and to eliminate the turbulence, which promote the deposit of impurities.
While the impurities having dimensions larger than the mesh void or larger than the perforation remain up-stream of the filtering surface, and the impurities of smaller dimensions pass through the mesh or slots, a quantity of particles having substantially the dimensions of the passage openings are caught between the wires, bars or edges of those openings, thus causing little, by little the clogging of any filter element despite cleaning.
There are also known filters of the counter-current cleaning type, formed by a helical spring having a length that may be varied; for example, the distance between turns, to change from the filtering mode, where the turns are close to each other; to the cleaning mode, where the turns are spaced apart from each other, and vice versa.
However, filters with this type of construction the distance between turns is constant and presents the short coming that it is not possible in practice, to obtain a distance beteen turns sufficient to provide for a given filtering degree. The present invention aims at remedying to the shortcomings which have just been mentioned, of the different types of known back-washing filters.